Signed for a not insignificant sum of £59 million, the 26-year-old Egyptian is thought to have penned a four-and-a-half year deal to keep him at the Etihad Stadium for what would be considered his peak years as a player.
The strength of his performances at Eintracht Frankfurt, where Marmoush really came into his own as a goalscorer and brilliant all-rounder, was arguably behind Pep Guardiola's insistence that the player was signed halfway through the 2024/25 season rather than in a chaotic double summer window on either side of the Club World Cup.
His 37 goals and 19 assists in 65 games for Eintracht were always going to make an underperforming Bundesliga outfit susceptible to outside interference.
Finishing sixth on 47 points in 2023/24 would seem more than acceptable on the face of it, although digging a little deeper into the detail reveals a starker truth.
Borussia Dortmund, who finished one place higher, were a cavernous 16 points better off, whilst champions Bayer Leverkusen's 90-point season saw them end the campaign 43 points ahead of Eintracht.
Of the top seven teams in the German top-flight that season, Frankfurt scored at least 15 fewer goals than any of the other six, and their solitary goal difference (+1) was 24 goals worse off than the next best team in this regard out of the top six.
Toppmoller's Eintracht side are flying this season
With Marmoush then being allowed to move on to pastures new halfway through the 2024/25 campaign, it appeared to be a foregone conclusion that Dino Toppmoller's side would crash and burn.
However, nothing could be further from the truth.
At the time of writing, a two-legged Europa League quarter-final against Tottenham Hotspur is there to be won, and Eintracht are sitting pretty in third place in this season's Bundesliga behind only Bayern Munich and Leverkusen.
If they can consolidate where they are in the remaining matches - as Bayern are already out of reach and Leverkusen soon will be - it will still be quite the remarkable campaign for the men from Frankfurt.
They've won more German top-flight games already this season (14) than they did in the entirety of the 23/24 campaign (11), and with 55 goals scored so far in the Bundesliga, only Bayern and Leverkusen have outscored them.
Incredibly, 52 of those goals have been scored from inside the box, suggesting that Toppmoller prefers some economy from his players when it comes to goals and not wasting chances.
A seven-game unbeaten run at the start of the calendar year is the first time they've achieved such a feat, too.
Whether by luck or judgment, Frankfurt's young players have really stepped up for their manager, and the results are there for all to see.
Hugo Ekitike, a striker once linked with multiple Premier League clubs, is currently one of the highest-ranked players in the Bundesliga in terms of xG (18.48), whilst 19-year-old Can Uzun has recently scored his fourth German top-flight goal.
The pace at which Eintracht transition from back to front has caused problems for many sides, both domestically and in Europe this season as well. That fearlessness of youth certainly has to play a part in the way the squad approach each game.
Gotze still contributing at the highest level
At the other end of the age scale is veteran German World Cup winner Mario Gotze.
Three goals and two assists already this season places him joint fourth in the entire Eintracht squad, and having played in six positions across the campaign, one can't really pigeonhole the 32-year-old.
Suffice to say that, after more than 300 Bundesliga games (and many more in Europe), Gotze still has the nous to ensure that his contribution is an important one.
One only needs to look at the passing accuracy across the squad to also understand that this is a team that likes to be in charge of the ball and move it about in their own time.
Many players have a pass completion percentage in the mid to high 80s, but Robin Koch has gone one better with 91.2% across the current season.
Defensively, they've been at it too, with Gotze's 40 tackles attempted the third highest in the squad behind 21-year-old Nathaniel Brown's 53 and Rasmus Kristensen's 49.
Another 21-year-old, Nnamdi Collins, doesn't seem to have minded getting his foot in either, with 22 tackles attempted, 15 of which were successful.
Marmoush moving on was best for all parties
Looking at things objectively, perhaps Marmoush moving on was actually the best for all parties.
It has allowed the player to spread his wings at a higher level and prove that he's able to do so, whilst Frankfurt now appear more creative and more willing to engage in other areas of the pitch, whereas before, the focus was, most likely, on playing Marmoush in at every opportunity.
Just as Paris Saint-Germain have found now that Kylian Mbappe has departed, less reliance on one player has allowed others to express themselves more fully.
They may not be the most celebrated of European teams at present, but carry on as they are doing, and it won't be long before Eintracht Frankfurt can be considered amongst Europe's elite.
